Transfer of magnetization from ¹H to half-integer quadrupolar nuclei in solids: comparison of different through-space...

The majority of chemical elements can be observed by NMR only through half-integer quadrupolar nuclei with spin S = 3/2, 5/2, 7/2 or 9/2. As a result, probing the proximities between these isotopes and protons is crucial for investigating the structure of numerous hydrogen-containing materials, including heterogeneous catalysts, pharmaceuticals, nanostructured materials, etc. These experiments must be conducted under magic-angle spinning (MAS) conditions to resolve the different chemical environments. In particular, different variants of the ¹H → S through-space refocused INEPT (D-RINEPT) sequence have been proposed to probe long-range proximities between protons and half-integer quadrupolar nuclei. These pulse sequences differ by the employed heteronuclear dipolar recoupling scheme as well as the method used to refocus the chemical shift anisotropy of protons during the defocusing and refocusing periods. Some of the most efficient D-RINEPT variants that have been reported so far, include (i) that employing the symmetry-based supercycled SR4₁² recoupling built from adiabatic inversion pulses incorporated into two spin-echoes and (ii) that using wPMRR (windowed Phase-Modulated Rotary Resonance) recoupling combined with two dipolar-echoes. In particular, the former has been used to enhance the sensitivity of NMR detection of quadrupolar isotopes via indirect DNP (Dynamic Nuclear Polarization) via protons. Nevertheless, there is a lack of comparative studies of these D-RINEPT variants in the literature. In the present study, we introduce new variants of this general scheme, exploring the incorporation of windowed basic elements into the R4₁² recoupling, and the application of these novel recoupling sequences into dipolar-echo instead of spin-echo refocusing. The efficiencies of these novel D-RINEPT sequences are compared with the existing ones as well as to that of CPMAS (Cross-Polarization under MAS). This experimental comparison is performed at 18.8 T with different MAS frequencies, ν_R = 10, 20, and 50 kHz, for ¹H → ²⁷Al and ¹H → ³⁵Cl polarization transfers in γ-Al₂O₃ and l-histidine·HCl·H₂O, respectively. The first sample features moderate ¹H-¹H dipolar interactions, whereas large ¹H-¹H couplings are present in the second. We demonstrate that at ν_R ≤ 20-25 kHz, the most efficient D-RINEPT variant is that employing on ¹H channel two spin-echoes incorporating (i) the SR4₁² recoupling built from adiabatic pulses, (ii) two composite pulses and (iii) continuous-wave (CW) irradiation during the windows. However, at ν_R = 50 kHz, the radiofrequency (rf) field requirement of adiabatic pulses is not compatible with rf power specifications of the probes, and the highest transfer efficiency is then achieved with (270₀90₁₈₀) composite π-pulses, instead of adiabatic ones in the SR4₁² recoupling. However, for γ-Al₂O₃, in which protons are subject to moderate homonuclear dipolar couplings, similar efficiencies are achieved when employing the R4₁²R4₁^-2 and SR4₁² recouplings built from windowed pulses with rf irradiation during half of the recoupling delays. These windowed recoupling sequences have the advantage of requiring less average rf power than their counterpart built from (270₀90₁₈₀) composite pulses. Compared to CPMAS transfers, the efficiencies of the best D-RINEPT variants are comparable, but they are simpler to optimize, more robust to offsets, and they can transfer to longer distances.